Hi.
Just ring up your network provider and ask for the NUC code, it should be free! If they try to charge you then it should be free! I used to work for Vodafone and you can get unlocking codes for free, depending on which advisor you get, some will try to charge you but just dispute it and they will do it for free. It doesn't cost the company anything to get your unlocking code so they shouldn't need to charge you!
Sometimes it can take a couple of days for the unlocking code to be received but once the advisor at your network provider requests the unlocking code for you, the code will then be updated onto your account, so if you ring up within a couple of days, they will guide you through unlocking your phone to all networks!
Just remember to either ring off someone else's phone or a landline because the advisor will talk you through unlocking the phone while you are speaking to them.
Hope this helps!